Two integrated partial repeats of simian virus 40 together code for a super-T antigen

Carcinogen-induced amplification of SV40 DNA inserted at 9q12-21.1 associated with chromosome breakage, deletions, and translocations in human uroepithelial cell transformation in vitro

The fate of integrated SV40 viral genome in SV40-immortalized human uroepithelial cells (SV-HUC) during multistep chemical transformation in vitro was studied. We previously reported that exposure of SV-HUC at passage (P) 15 to the chemical carcinogens 3-methylcholanthrene (MCA), 4-aminobiphenyl (ABP), or the N-hydroxy metabolites of ABP causes tumorigenic transformation and/or neoplastic progression. We report now that these same chemical carcinogens induce amplification of SV40 DNA in SV-HUC. We used fluorescence in situ hybridization (FISH) to show that this amplification occurs at the SV40 integration site, which was mapped near a common fragile site at 9q12-21.1 on the der(9)t(8;9) chromosome that is present in all SV-HUC at the earliest passage studied. Karyotypic analysis, along with FISH, also revealed that all carcinogen-induced tumors (T-SV-HUCs) had breaks at 9q12-21.1, deletions of 9q12-21.1-->pter, and new derivative chromosomes containing SV40 in the segment 9q12-21.1-->9q34::8q22-->8qter. Southern blot analysis, along with FISH, confirmed SV40 genome rearrangements in T-SV-HUCs. In contrast, no 9q12-21.1 breaks were observed in control SV-HUC. Thus, these results associate 9q12-21.1-->pter alterations with HUC tumorigenic transformation. In addition, these results indicate for the first time that (carcinogen-induced) amplification of chromosome-integrated viral genes may create sites that are prone to breakage, deletions, and translocations. These results suggest a new mechanism by which chemical carcinogens in synergy with a DNA tumor virus could initiate a cascade of events that contribute to the genomic instability associated with tumorigenesis.

Simian virus 40 (SV40) T-antigen mutations in tumorigenic transformation of SV40-immortalized human uroepithelial cells

pSV2Neo, a plasmid that contains the wild-type simian virus 40 (SV40) origin of replication (ori), is widely used in mammalian cell transfection experiments. We observed that pSV2Neo transforms two nontumorigenic SV40-immortalized human uroepithelial cell lines (SV-HUC and CK/SV-HUC2) to G418 resistance (G418r) at a frequency lower than that at which it transforms SV-HUC tumorigenic derivatives (T-SV-HUC). Transient expression studies with the chloramphenicol transferase assay showed that these differences could not be explained by differences in Neo gene expression. However, when we replaced the SV40 ori in pSV2Neo with a replication-defective ori to generate G13.1Neo and G13.1'Neo, the G418r transformation frequency of the SV40-immortalized cell lines was elevated. Because SV40 T antigen stimulates replication at its ori, we tested plasmid replication in these transfected cell lines. The immortalized cell lines that showed low G418r transformation frequencies after transfection with pSV2Neo showed high levels of plasmid replication, while the T-SV-HUC that showed high G418r transformation frequencies failed to replicate pSV2Neo. To determine whether differences in the status of the T-antigen gene contributed to the phenomenon, we characterized the T-antigen gene in these cell lines. The results showed that the T-SV-HUC had sustained mutations in the T-antigen gene that would interfere with the ability of the T antigen to stimulate replication at its ori. Most T-SV-HUC contained a super-T-antigen replication-defective ori that apparently resulted from the partial duplication of SV40 early genes, but one T-SV-HUC had a point mutation in the ori DNA-binding domain of the T-antigen gene. These results correlate with the high G418r transformation frequencies with pSV2Neo in T-SV-HUC compared with SV-HUC and CK/SV-HUC2. Furthermore, these results suggest that alterations in SV40 T antigen may be important in stabilizing human cells immortalized by SV40 genes that contain the wild-type SV40 ori, thus contributing to tumorigenic transformation. This is the first report of a super T antigen occurring in human SV40-transformed cells.

PAb 2000 specifically recognizes the large T and small t proteins of JC virus

A monoclonal antibody, PAb 2000, has been derived which recognizes the large T protein of JC virus (JCV), but not the corresponding proteins of the related polyomaviruses BK virus (BKV) and SV40. The epitope bound by PAb 2000 was localized to the amino-terminal 81 amino acids of this multifunctional protein. As observed previously with several monoclonal antibodies that bind a similar region of SV40 large T antigen, PAb 2000 was found to interact with the small t antigen and the denatured form of large T antigen. This monoclonal antibody recognized a subpopulation of T protein, the abundance of which varied in different species of cells transformed by JCV. The availability of PAb 2000, the first JCV T antigen-specific monoclonal antibody, will facilitate the purification and biochemical characterization of the JCV oncoproteins.

Incomplete splicing of simian virus 40 large T antigen transcripts in a transformed mouse cell line

The SV40 large T antigen transcripts from an SV40-transformed mouse embryo fibroblast cell line (215) have been analyzed by Northern blots and by mapping and sequencing of the corresponding cDNAs. We have observed that T antigen mRNA is highly overexpressed in the 215 cells, and is mostly unspliced, but there are no sequence changes at the splice sites. However, normal amounts of wild-type T antigen are produced, suggesting that the splicing and translation of T antigen RNA is tightly controlled in these cells.

A stable flank of unstable lymphotropic papovavirus integration sites is associated with a cellular S1 nuclease-sensitive sequence

Integration of viral DNA sequences is associated with stable transformation by viruses of the polyomavirus genus. In LPV-HE cells, a hamster embryonal cell line transformed by the African green monkey lymphotropic papovavirus (LPV), viral integration is unstable during tissue culture passage, leading to subclones with rearrangements of the original integration locus. Three of four viral integrations that were molecularly cloned together with flanking cellular sequences are identical in one junction but are different from each other at the other flank. The "stable" flank which is already present in early passage cells contains an S1 nuclease-sensitive sequence located approximately 60 bp outside of the viral-cellular junction. The presence of this site may contribute to the stability of the flanking region.

Spontaneous rearrangement of integrated simian virus 40 DNA in nine transformed rodent cell lines

Frequencies of spontaneous DNA rearrangement within or near integrated simian virus 40 (SV40) DNA were measured in four transformed mouse and rat cell lines of independent origin and in five clones of the SV40-transformed mouse line SVT2. Rearrangements were detected as polymorphisms of restriction enzyme fragment length in subclones of the lines. At least 17% of the subclones of each line had detectable rearrangements. The rate of rearrangement was calculated to be at least 5 x 10(-3) events per cell per division. No rearrangements were detected in sequences of an immunoglobulin gene, part of the coding region of the mouse protein p53, and five proto-oncogenes. The possible role of recombination between duplicated segments of integrated SV40 DNA in generating rearrangements was studied in the five SVT2 clones, which differed in the number of duplications within a single SV40 DNA segment. The SVT2 clone that had no duplications, M3, became rearranged further at least as frequently as did closely related lines with one, two, or three duplications. Another line in this group that had one small duplication, X1, had a much higher frequency of rearrangement than did the others; integrated SV40 DNA of X1 became mostly rearranged within 100 cell divisions. The examples of M3 and X1 suggested that the high rate of rearrangement characteristic of integrated SV40 DNA was influenced more by the presence of particular sequences within or near integrated SV40 DNA than by the number or extent of duplicated sequences.